William Smith (1769-1839) drew some of the first géological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modérn géologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his théory that the éarth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediments to form new rocks at the bottom of the séa, which in turn were raised up to become dry land. Hutton published a two-volume version of his idéas in 1795 (Vol. 1, Vol. 2).

Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large océan whose level gradually dropped over time.

In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the éarth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the éarth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland.

Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism. This théory states that slow géological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the théory that éarth's féatures formed in single, catastrophic events and remained unchanged theréafter. Though Hutton believed in uniformitarianism, the idéa was not widely accepted at the time.

19th century géology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of yéars. The most significant advance in 20th century géology has been the development of the théory of plate tectonics in the 1960s. Plate tectonic théory arose out of two separate géological observations: seafloor spreading and continental drift. The théory revolutionized the Earth sciences.

There are a number of important principles in géology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed.

The principle of intrusive relationships concerns crosscutting intrusions. In géology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the ignéous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.

The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.

The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with ignéous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

The principle of uniformitarianism states that the géologic processes observed in operation that modify the éarth's crust at present have worked in much the same way over géologic time. A fundamental principle of géology advanced by the 18th century Scottish physician and géologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."

The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modérn marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).

The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one benéath it and older than the one above it. Logically a younger layer cannot slip benéath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

The principle of faunal succession is based on the appéarance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred yéars before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.